1. Field of the Invention
The present invention relates to a cubic boron nitride based sintered material effectively utilizable as the material of a tool for high speed cutting spheroidal graphite cast iron, etc., and a production method of this cubic boron nitride based sintered material.
2. Description of the Related Arts
The spheroidal graphite cast iron is cast iron in which spheroidal graphite is crystallized by adding cerium (Ce) or magnesium (Mg) (or a Mg alloy) into a molten bath of low sulfur (S) (&lt;0.02%) and low phosphorus (P) (&lt;0.2%). The spheroidal graphite cast iron has high tensile strength and relatively large tenacity in comparison with common cast iron. Therefore, the spheroidal graphite cast iron is widely used as a material of mechanical parts requiring strength and is also used in many cases as trunk important parts mainly constituting an automobile by its recent higher performance.
Cutting work of the spheroidal graphite cast iron is normally required after casting to set this spheroidal graphite cast iron to have a final shape size of the above trunk important parts, etc. A cutting work tool of the spheroidal graphite cast iron must have performance capable of rapidly processing this spheroidal graphite cast iron without any waste with required processing accuracy. When a tool edge is worn and damaged by chipping, etc., fins are caused on a processing surface of the spheroidal graphite cast iron, etc. so that no required size accuracy and surface roughness are obtained. Accordingly, a defective product is formed so that this defective product cannot be forwarded as produced goods.
Therefore, when the above tool wearing and chipping damage, etc. are caused, the cutting work tool must be immediately exchanged. This tool exchange must be reduced as much as possible since this tool exchange causes a reduction in productivity.
Accordingly, a cutting work tool having a long life and undamaged by chipping, etc. without any wearing of the above tool edge is strongly desired to cut the spheroidal graphite cast iron at high speed.
For example, a ceramic sintered body constructed by TiC, Al.sub.2 O.sub.3 and SiC whiskers as described in Japanese Published Patent No. 8-16028 is proposed as a cutting work tool for solving the above disadvantages.
As described in Japanese Published Patent No. 64-4986, a cubic boron nitride based sintered material having a binding material constructed by Ti.sub.2 AlN and one or two kinds among Si.sub.3 N.sub.4 and Al.sub.2 O.sub.3 is proposed as a cutting work tool for cast iron. Further, Japanese Published Patent No. 64-4987 proposes a cubic boron nitride based sintered body characterized in that Si.sub.3 N.sub.4, Si.sub.2 W and Ti.sub.2 AlN constitute a binding material.
However, there are the following problems with respect to a cubic boron nitride based sintered material used in the above conventional cutting tool for the spheroidal graphite cast iron, etc.
Namely, no cubic boron nitride is included in the ceramic sintered body constructed by the above TiC, Al.sub.2 O.sub.3 and SiC whiskers. Therefore, there is a merit in that the ceramic sintered body can be cheaply produced. However, no ceramic sintered body can have durability for realizing a long life at a desired level.
In contrast to this, it is considered that the above cubic boron nitride based sintered body shows excellent performance with respect to general cast iron in comparison with the ceramic sintered body including the above SiC whiskers.
However, the above conventional cubic boron nitride based sintered body shows the excellent performance with respect to the general cast iron, but no satisfied durability can yet be obtained with respect to the spheroidal graphite cast iron including magnesium (Mg) as an active metal and difficult to be processed.